The science of metallography is essentially the study of the structural characteristics or constitution of a metal or an alloy in relation to its physical and mechanical properties. One important phase of this study is known as macroscopic examination and involves the visual observation of the rather gross structural details of a metal, either by the unaided eye or with the aid of a low-power microscope or binocular. Because the attending magnifications are of low order, usually under 10.times., macroscopic observations are somewhat limited as to the kind of metallurgical data revealed. However, macroscopic examinations, when appropriately carried out, are of considerable importance in many instances with some metallic characteristics preferably determined by such studies.
Another phase of metallography deals with the microscopic examination of a prepared metal specimen, employing magnifications with the optical microscope of from 100.times. to as high as 500.times.. Such microscopic studies are of much broader scope than are macroscopic examinations, and under appropriate conditions of observation there will be revealed to the trained metallographer an abundance of constitutional information concerning the metal or alloy under investigation, such as, for example, grain size; the size, shape, and distribution of secondary phases and nonmetallic inclusions; and segregation and other heterogeneous conditions -- all of which profoundly influence the mechanical properties and behavior characteristics of the metal. When these and other constitutional features are determined by microscopic examination and the extent to which they exist in the microstructure is known, it is then possible to predict with considerable accuracy the expected behavior of the metal when used for a specific purpose. Of equal importance is the fact that within limits there is reflected in the microstructure an almost complete history of the mechanical and thermal treatment that a metal has received.
The surface of the metal which is to be so examined is first prepared according to more or less rigid and precise procedures. With the use of the modern metallurgical microscope and precision optical parts where the obtainable resolution may be as great as a fraction of the wave length of light used to illuminate the specimen, it is evident that specimen preparation is of great importance. Improper preparation is likely to remove all-important inclusions, erode grain boundaries, or temper hardened steel specimens, ultimately producing a structure, superficially at least, which upon microscopic examination will appear entirely different from that which is truly representative and characteristic of the metal. Obviously, an examination of such a prepared specimen will lead only to erroneous interpretations and unreliable conclusions.
In general, specimen preparation procedure consists of first obtaining a flat, semipolished surface by various methods known to those skilled in the art, such as, chemical mechanical polishing and electro-mechanical polishing. These operations ultimately produce the flat, scratch-free, mirror like surface which is required before the specimen can be etched and the metallographic structure appropriately revealed.
Metallographic etching is then used to reveal particular structural characteristics of a metal that are not evident in the as-polished condition.
Two popular etching techniques known to those skilled in the art include chemical etching and electrolytic etching. Although the manipulations involved in etching a metallographic specimen are relatively simple to carry out, a certain amount of skill is required on the part of the technician to secure a satisfactory etched surface.
Perhaps the most important preliminary consideration in the procedure is the selection of an appropriate etching reagent from the many that are recommended for any given metal or alloy. This selection requires judgment and knowledge of the behavior of the various reagents when used under recommended conditions. A selected agent must be used for the specific purpose for which it was intended, and to secure the desired results, the directions pertaining to its use must be adhered to.
In the art of electrolytic or chemical etching, the selection of an appropriate etching reagent depends primarily upon the composition of the metal or alloy to be etched, the method of etching to be utilized and which constituents in the structure are to be revealed by etching. Thus each metal or alloy has a number of recommended etchants, the use of which is quite specific.
It is generally known to chemically etch phosphorus-containing gray irons, to reveal the grain cells, with Stead's reagent for both macroscopic and microscopic examination. Stead's reagent (which is composed of 1.2 weight percent cupic chloride, 4.6 weight percent magnesium chloride, 2.8 weight percent hydrochloric acid and 91.4 weight percent ethanol) selectively darkens low phosphorus regions in the iron, leaving high phosphorus regions (normally segregated at grain cell boundaries) unattached and light.
The technical literature indicates that chemical etching of gray irons with Stead's reagent requires immersion of the sample in the reagent for up to three hours. It has been found, however, that chemical etching of phosphorus-containing gray irons can be performed in much shorter times, e.g., from about 1 to about 5, generally about 3 to about 4 minutes. Although the reduced immersion times are of considerable significance in commercial foundry operations, there are a number of other problems with chemical etching of phosphorus-containing gray irons with Stead's reagent.
Chemical etching of phosphorus-containing gray irons with Stead's reagent has been found to be generally more successful when the gray iron has a high phosphorus content, e.g., about 0.5 to 0.6 weight percent or more. When the phosphorus content of the gray iron is low, e.g., about 0.06 weight percent or less, the etched surface has been often found to be obscure with relatively poor (if any) differentiation of the grain cells. The obscurity has been found to remain even with longer etching times of 10 to 15 minutes or more. In such cases, visual and microscopic observations of grain cell size and count are difficult, if not impossible, to perform and photographs and photomicrographs are also too obscure to be of value to the investigator. Although the problem is more acute with low phosphorus-containing gray irons, it often occurs also in the chemical etching of gray irons with intermediate (e.g., 0.07 to 0.15) and high (e.g., 0.16 and above) phosphorus-containing gray irons. Other types of chemical etching (e.g., heat tinting and deep acid etch) have also been found to be generally unsuitable for consistent clear and definite etching of the various phosphorus-containing gray irons.